When gas is filled into a pressurized tank, the compression heat from the compression of the gas causes the temperature of the gas inside the tank and, consequently, also the temperature of the tank material to increase significantly.
Such gas tanks typically have a certain upper temperature limit. For instance, a hydrogen tank for use in a vehicle normally has an upper temperature limit of 85° C., which may not be exceeded by the hydrogen temperature inside the tank at any time during the refueling of the tank.
The increase of the temperature of the gas inside the tank is highly dependent on the refueling time, i.e. time used for filling the tank. The longer the refueling time, the more heat energy will be transferred from the gas inside the tank through the tank material to the environments during the refueling. Consequently, a fast refueling results in a higher gas temperature inside the gas tank than a slow refueling.
In order to avoid that the upper temperature limit is exceeded, the refueling time is normally adjusted so that the upper temperature limit will only be reached in the most extreme case, known as the “hot case”. Thus, if the conditions in the gas tank are not extreme, the temperature of the gas therein will stay below the upper temperature limit at any time during the refueling. The “hot case” conditions are normally calculated from a model taking into consideration different parameters of which the ambient temperature is one of the most important.
For a gas tank with a fixed volume, the density (ρ) of the gas therein is directly proportional to the mass of the gas and, thereby, to the amount of gas in the tank. The density of a given type of gas depends only on the pressure (P) and the temperature (T) of the gas.
Thus, when the temperature and the pressure of the gas within a tank of a well-defined volume is known, the density of the gas and, thereby, also the amount of gas in the tank is completely defined. This means that, if the refueling is stopped at a certain target pressure, the amount of gas in the tank depends on the temperature of the gas.
The term “State Of Charge” (SOC) is normally used for quantifying the actual amount of gas inside a gas tank. SOC is defined as the ratio between the actual gas density and a nominal density, where the nominal density is the density at the Nominal Working Pressure (NWP) at a certain reference temperature (typically 15° C.). Thus
                              SOC          ⁡                      [            %            ]                          =                            ⁢                                                    ρ                                  P                  ,                  T                                                            ρ                                  NWP                  ,                                      15                    ⁢                    °                    ⁢                                                                                  ⁢                                          C                      .                                                                                            ·            100                                              (        1        )            
This equation states that the gas tank is full, if the gas density inside the tank equals the nominal density. If SOC exceeds 100%, the tank is overfilled.
In refueling situations at which the vehicle does not communicate the gas tank pressure and temperature to the refueling station, a suitable stop criterion is needed in order to stop the refueling process at an appropriate stage. Typically, a predefined target pressure is used to stop the refueling before the vehicle tank is overfilled. This target pressure can be defined in several different ways, but typically it depends on the ambient temperature and the start pressure within the gas tank before the refueling. Due to the lack of communication from the vehicle, the tank temperature is unknown, which means that the SOC will also be unknown to a certain extent, even if the refueling is stopped exactly at the predefined target pressure.
The target pressure is normally defined by calculations and/or measurements prior to the refueling. This means that a number of assumptions, including the refueling time and corresponding refueling time tolerances, have to be made when defining the target pressure. When calculating the target pressure, the so-called “cold case” conditions are used. This means that the target pressure is defined as the pressure which, at the gas temperature resulting from the longest possible refueling time within the defined tolerances, results in an SOC of 100%. If the refueling takes place faster than defined by the “cold case” conditions, the increase of the gas temperature will be larger, and target pressure will be reached and the refueling will be stopped before the SOC reaches 100%.
As long as the actual refueling time corresponds to the assumed refueling time within the corresponding tolerances, a safe refueling with a gas temperature below the upper temperature limit and an SOC not exceeding 100% is ensured. If, however, the refueling time falls outside the tolerances, problems may occur. If the refueling time is too short, the upper temperature limit may be exceeded, and if the refueling time is too long, the tank may be overfilled.
This means that, if the refueling is too slow, it has to be stopped before the target pressure has been reached because there is a risk of overfilling the vehicle tank. This is unfortunate and inconvenient for the customer, who would normally rather experience a slow refueling than not having his vehicle refueled at all.
One of the most likely reasons for slow refueling with the refueling time exceeding the tolerances is low refueling station capacity. Consequently, there is typically a lower limit for the refueling station capacity, below which the refueling station is not able to perform a refueling within the defined refueling time tolerances, and a certain part of the refueling station capacity cannot be utilized. This is both an inefficient and unnecessarily expensive situation.